In contrast to post-coating where the coating is carried out after first fabricating in the form of the target manufactured product, precoated steel sheet is metal sheet (cut sheet or coil) which is subjected to coating beforehand prior to the fabrication thereof, and it is widely employed for building materials such as roofs and walls, and for the housings of audio equipment, refrigerators, partitions, and the like, in that there are advantages in terms of cost, energy savings, and materials savings when compared to post-coating.
The coating of precoated steel sheet is generally carried out at both the upper and underside faces thereof, where the upper face side is the region forming the outer face of the product following fabrication, so there is applied thereto a coating material which is outstanding in terms of coated film external appearance, physical properties (hardness, adhesion, scratch resistance, bendability, etc.), corrosion resistance, and weatherability, etc., and the coating process adopted therefor is a 2-coat 2-bake system employing a primer and topcoat coating material, or a 3-coat 3-bake system employing a primer, intermediate-coat coating material, and a topcoat coating material. On the other hand, since the underside face is positioned on the inside of the product following fabrication, the same level of coated film external appearance and weatherability is not demanded as in the case of the upper face side, so, from the point of view of cost, the coating thereof is generally carried out by a 1-coat 1-bake system employing just a backcoat coating material, but in applications where corrosion resistance is especially demanded a 2-coat 2-bake system employing a primer and a backcoat coating material is adopted.
While, the upper and underside faces of the precoated steel ideally have the same appearance (gloss, color tone), since the underside of the precoated steel forms the inside of the processed product following fabrication it is hardly ever seen, so a high level of design is not demanded and usually just, for example, two types of coating material are provided as the backcoat coating material, namely a high-gloss backcoat coating material and a low-gloss backcoat coating material, such that in the case where the upper side gloss is comparatively high the high-gloss backcoat coating material is applied to the underside, whereas in the case where the upper side gloss is comparatively low the low-gloss backcoat coating material is applied to the underside, and, in this way, the number of coating material products employed is reduced and costs improved.
Of these, the low-gloss backcoat coating material has hitherto been produced by incorporating an inorganic powder such as silica which has a gloss-lowering effect, but problems have arisen such as a lowering in coatability caused by a rise in viscosity, or a reduction in bendability, and also gloss transfer due to poor pressure mark resistance, etc. Here, gloss transfer due to poor pressure mark resistance refers to the phenomenon whereby, in the case where precoated steel sheet with a different gloss at the upper side from that at the underside has a coil shape, and strong pressure is applied while the upper side and underside are in contact, the gloss of the high-gloss face is lowered while, conversely, the gloss of the low-gloss face is raised, and this is perceived as an apparent transfer of gloss from the high-gloss side to the low-gloss side. This phenomenon is thought to occur because the low-gloss face, which has minute projections and indentations, is pressed against the comparatively smooth high-gloss face, so that these minute projections and indentations on the low-gloss face eat into the surface of the high-gloss face and lower the smoothness thereof, while, at the same time, the fine projections and indentations on the low-gloss face are squashed against the smooth high-gloss face and thereby smoothened. When this phenomenon occurs, the precoated steel sheet surface gloss changes while stored in the coiled state, and there is also an especially marked change in gloss in regions where there is a localized load application, with the result that gloss transfer variations arise where the gloss transfer is unevenly manifested, so there is the problem that it is not possible to provide a uniform final product.
In patent document 1 there is disclosed a backcoat coating material composition which contains at least one type of fine resin particles selected from polyamide resins, polyacrylonitrile resins, and acrylic resins.
However, there are problems therewith in that, depending on the type of fine resin particles employed, it fails the pressure mark resistance test and shedding of the fine particles themselves takes place.
In patent document 2, there is described a means for enhancing the scratch resistance of steel sheet by using soft urethane beads, but when the precoated steel sheet is subjected to high pressure the beads themselves are distorted and it is not possible to sustain the expected performance.
In patent document 3, reactive type urethane beads are described. The characteristic feature of these beads is that they have a reactive blocked isocyanate at the surface but, in practice, good results are not shown when a pressure mark resistance test is carried out. The reason for this is thought to be that, while shedding of the beads due to rubbing, etc., is indeed suppressed by chemical bonding of the reactive urethane beads to the matrix resin, when the beads are subjected to a deforming pressure they do not then return to their original state.